1. Field of the Invention
The present invention relates to a music reproducing device that outputs an audio signal to a headphone.
2. Description of the Related Art
In a headphone output of a music reproducing device that outputs an audio signal to a headphone, there are systems that are called unbalanced and balanced (for example, see JP 2013-005291 A). In an unbalanced system, a three-pole terminal with a diameter of 3.5 mm is used, and the audio signal is transmitted by two kinds of “hot” and “cold”. Meanwhile, in a balanced system, a four-pole terminal with a diameter of 2.5 mm is used, and a signal is transmitted by three kinds of “ground”, “hot” and “cold”. “Cold” is opposite phase of “hot”. In case that external noise occurs, noise of the same phase is superposed on both “cold” and “hot”. By inverting phase of “cold” and mixing inverted “cold” signal with “hot” signal, external noise is cancelled and amplitude of the audio signal becomes twice. For this reason, the balanced system is strong against noise and sound quality is good in the balanced system.
The music reproducing device having the balanced output includes two blocks. Each of two blocks is configured by a 2 channels D/A converter (hereinafter, referred as to “DAC”) and a 2 channels amplification circuit. Each of two blocks is used for an L channel audio signal and an R channel audio signal. The music reproducing device including the balanced output includes a standard unbalanced output because the balanced system is not standard. A circuit for the balanced output and the unbalanced output is designed in common from the point of view of cost. Namely, each of two blocks is also used for an L channel audio signal and an R channel audio signal in the unbalanced output.
A digital audio signal is input to the DAC with I2S system. The L channel audio signal and the inverted L channel audio signal are input to the DAC for the L channel audio signal. Further, The R channel audio signal and the inverted R channel audio signal are input to the DAC for the R channel audio signal. Therefore, signal transmission is 4 channels, and there is a problem that each of signal output and wiring is many.